Hardening treatment is performed on the surface of a steel sheet in order to improve wear resistance and fatigue strength of steel material parts. A known example of such hardening treatment is thermal treatment in a controlled atmosphere, such as carburizing treatment, nitriding treatment, or softnitriding treatment.
When hardening treatment is performed on the steel sheet surface, the steel sheet surface hardens, whereas heating in the hardening treatment causes crystal grains in a sheet-thickness central portion of the steel sheet to grow and coarsen, softening the hardness (strength) of the sheet-thickness central portion.
A known means for suppressing the growth of crystal grains in the sheet-thickness central portion is to add a small amount of Nb. When Nb is added to steel, niobium carbide (NbC; a precipitate in which Nb and carbon are bound together) is precipitated, and this NbC has a pinning action of suppressing the growth of crystal grains, which is presumed to prevent the growth of crystal grains in the sheet-thickness central portion in thermal treatment (e.g., see Patent Literature 1).